Atomistic Simulation Of The Interaction Between The Defects And The Cu Rich Precipitates In α-Fe

Precipitation strengthening is one of the important methods to enhance mechanical properties of steels,and the precipitates have a significant influence on the material properties such as plasticity and strength,grain growth and recrystallization,and fracture behavior.The low solubility of Cu in Fe matrix leads to the precipitation of copper and the Cu precipitates are the common the strengthen phase in steels.In new high-strength low-alloyed steels,the precipitation strengthening and the grain refinement strengthening induced by Cu rich precipitates are the primary reason for the extraordinary mechanical strength.Cu rich precipitates are also used for improving the fracture resistance.The macroscopic phenomenon is determined by the microscopic mechanism and the properties of steels with Cu rich precipitates are results from the interaction between Cu rich precipitates and the defects in the Fe matrix.The strength is related to the dislocation motion resistance of precipitates,grain refinement is related to the pinning effect of precipitates on grain boundary(GB)migration,and fracture property is related to the impact of precipitates on crack propagation.Therefore,the study of the defects motion and the interaction of defects and Cu rich precipitates in α-Fe contributes to understand the macroscopic property and design steels more suitable for application.As a simulation method in micro and nano scale,Molecular Dynamic(MD)simulation is widely used to verify and supplement the theory and the experiments results and has been an important method for the research of microscopic mechanism in materials.In this work,the dislocation slipping motion,the shear-coupled grain boundary motion(SCGBM),the crack propagation and the interaction of Cu rich precipitates and the above defects in α-Fe were investigated using MD simulation.The influence of simulation parameters such as temperature,size and constituent of Cu rich precipitates on the defects motion or interaction has been studied.The main research contents and conclusions are as following:(1)In the dislocation part,the slipping motion of edge and screw dislocation was firstly simulated in α-Fe and the influences of temperature and applied stress were studied.Computational results show that the slipping velocity is constant under certain temperature and applied stress conditions for both edge and screw dislocation.The velocity increases with the increase of the applied stress at one applied temperature.The effect of temperature on the edge and screw dislocation slipping velocity was opposite.The velocity of edge dislocation is in the viscous region and the velocity of screw dislocation is in the thermally activated region.The interaction of dislocation and Cu rich precipitates were performed and it is found that the dislocation stress field assists a structure transformation of the precipitates from BCC to FCC/HCP during the gliding of the dislocation.For the Cu precipitates with diameters less than 4 nm,most of the atoms returned to BCC structure after the dislocation leaves away.The structure transformation of Cu Fe precipitates is identical with pure Cu precipitates with diameters less than 4nm.The outer Ni shells contribute the precipitate transformation to FCC/HCP structure and reduce the critical diameter of structure transformation.The critical stress has been calculated for the precipitates and it is demonstrated that the FCC/HCP fraction of the precipitate contributes to the critical stress in this interaction.(2)In the grain boundary part,MD simulation has been performed to investigate the SCGBM of two [110] symmetric tilt GBs,Σ9[110](221)and Σ17[110](223),in α-iron and the effects of temperature and strain rate on SCGBM have been studied.Computational results showed that the migration directions of two grain boundaries were opposite and strain rate has almost no effect on the SCGBM.At high temperature,the GB sliding is investigated in the SCGBM for Σ9[110](221).The coupling factor βwhich is defined as the ratio of the velocities of GB lateral translation and migration has been calculated.A geometric model of β dependent on misorientation angle is constructed in [110] symmetric tilt GBs of BCC metals.The model is branched into two modes(<100> and <111>)corresponding to the perfect dislocation burgers vectors in BCC metals.The β values calculated in <111> mode show good agreements with MD simulation results for both two GBs.The atomistic mechanisms of the SCGBM processes are also investigated.A same structural unit transformation is observed for the two GBs,which confirms that both Σ9[110](221)and Σ17[110](223)GBs move in<111> mode during the SCGBM process.The interaction of SCGBM and Cu rich precipitates were performed.It is found that the SCGBM is pinned by the precipitates and the critical shear stress in the interaction increases with precipitate size.The change of precipitate shape from sphere to ellipsoid and structure transformation from BCC to FCC/HCP/unknown are investigated during the interaction.It is similar with the dislocation interaction that the atoms return to BCC structure after the interaction for the precipitates with 1-3 nm diameter.The outer Ni shells in Cu Ni mixed precipitates contribute the pinning for the SCGBM.(3)In crack part,the crack propagation under mode Ⅰ loading for [100](001),[110](001),[111](112)cracks was performed using MD simulation,and the influence of temperature in [100](001)crack propagation was also studied.Computational results show that the twinning mechanism is investigated to release stress of crack tip for all three cracks.The [110](110)twinning is investigated [100](001)crack,and [111](112)twinning is investigated in [110](001)and [111](112)cracks,respectively.Step shaped twinning boundary induce by the double cross slip of dislocation is investigated in[100](001)crack propagation at high temperature.The peak stress in crack propagation decreases with the temperature.The interaction of [100](001)crack and Cu rich precipitate were simulated and the influence of precipitate position(on the pre-crack plane and on the twinning plane)on the interaction has been studied.The twinning is effected by the precipitate and the slipping plane of partial dislocation changes during the interaction.Comparing the crack length for precipitates on the two position,the precipitates on the pre-crack plane show better fraction resistance in the crack propagation.The different influence of Cu precipitates on fracture property between the RPV steel and the steel containing copper is interpreted in nanoscale.The lower size of Cu precipitates and the high service temperature are the main reason for the embrittlement induced by Cu precipitates.